1. Field of the Invention
The present invention relates to a cell selection scheme for a CDMA (Code Division Multiple Access) mobile communication system in which a plurality of base stations provided in respective cells transmit perch channels which are spread by using mutually different spread codes assigned to these base stations and each mobile station judges a located cell by receiving the perch channels.
2. Description of the Background Art
Here, a conventional cell selection method will be described for an exemplary case of using the conventional digital mobile communication system called PDC (Personal Digital Cellular) system which is currently providing a mobile communication service in Japan.
In the conventional cell selection method, each one of a plurality of base stations provided in respective cells continuously transmits the perch channel at a constant transmission power. Here, the constant transmission power may be different at different base stations.
Then, in order to carry out a located cell judgement, the mobile station measures a receiving level of each perch channel transmitted from each base station, and judges that it is currently located in a cell of a base station which transmitted the perch channel with the largest receiving level.
FIG. 1 shows an exemplary relationship between a mobile station position and a perch channel receiving level. Here, it is assumed that the mobile station is located on a straight line joining two base stations, and in FIG. 1, a horizontal axis represents a position of the mobile station between these two base stations and a vertical axis represents a receiving level value for a perch channel from each base station at each position. Also, FIG. 1 shows an exemplary case in which the transmission powers used at the base station BS1 and the base station BS2 are different.
As shown in FIG. 1, the perch channel receiving level decreases exponentially as the mobile station moves away from the base station. Note that, in practice, the perch channel receiving level has more complicated minute variations due to the presence of attenuation factors other than a distance from the base station, but these other attenuation factors are ignored here for the sake of simplicity. When the mobile station is located at a position X, for example, the receiving level of the perch channel from the base station BS2 is greater than the receiving level of the perch channel from the base station BS1 so that the mobile station judges that it is currently located in a cell of the base station BS2.
When this conventional cell selection method is applied to the CDMA mobile communication system, the degradation of the system capacity will be caused for the following reasons, especially when the perch channel transmission powers are different at different base stations as in an exemplary case shown in FIG. 1.
FIG. 2 shows an exemplary relationship between a mobile station position and a transmission attenuation value. In FIG. 2, the horizontal axis represents a position of the mobile station between two base stations BS1 and BS2 and the vertical axis represents a transmission attenuation value. This relationship of FIG. 2 is applicable not only to the perch channel but also to the communication channels in both an upward direction (from a mobile station to a base station) and a downward direction (from a base station to a mobile station). As shown in FIG. 2, the transmission attenuation increases as the mobile station moves away from the base station. A value of the transmission attenuation depends on a distance from the base station regardless of which base station it is, so that the transmission attenuation value for the base station BS1 and the transmission attenuation value for the base station BS2 become equal at a midpoint between the base station BS1 and the base station BS2.
When the cell judgement according to the perch channel receiving level is carried out similarly as in the conventional cell selection method, the mobile station located at a position X will judge a cell of the base station BS2 as a located cell as described above. However, as can be seen from FIG. 2, the position X is located closer to the base station BS1 so that the transmission attenuation value for the base station BS1 is smaller than that for the base station BS2.
Now, in the CDMA mobile communication system, the system capacity is determined by an amount of interference power. In order to prevent a decrease of the system capacity, it is necessary to reduce an amount of interference power, and in order to reduce an amount of interference power, it is necessary to make the transmission power at the a transmitting side as small as possible within a limit of maintaining a required receiving quality at a receiving side. Then, in order to reduce the transmission power while maintaining the required receiving quality at a receiving side, it is necessary for the mobile station to carry out the communication with a base station which has the smallest transmission attenuation value.
In this regard, according to the conventional cell selection method, a cell of a base station with a larger transmission attenuation value can be judged as a located cell as in a case of the cell judgement at a position X of FIG. 1 and FIG. 2. Consequently, the transmission power will be increased when the communication between the mobile station and the base station is started in such a cell, and therefore there has been a problem that the degradation of the system capacity can be caused.
The above problem arises when the perch channel transmission powers are different at different base stations. However, in the current mobile communication service, there is a need to use a simplified base station especially in a case of providing micro-cells within a macro-cell or in a case of supporting an indoor communication, so that a use of such a simplified base station imposes a limitation on an available transmission power due to the base station equipment. When a presence of such a simplified base station is properly taken into consideration, it is expected that a situation in which the perch channel transmission powers are different at different base stations can be encountered quite frequently, so that this problem can be a serious problem in a case of realizing the current mobile communication service by using the CDMA scheme.